In the manufacture of electric cables, particularly those having a conductor insulated with a multi-layer covering, for use at elevated temperatures under extreme pressure and exposure to oil and water environments, it has been found useful to employ an insulation comprising a thermosetting rubber containing ethylene and propylene usually referred to as an "ethylene-propylene rubber" or "EP rubbers". The ethylene- propylene rubber may be a copolymer, though more commonly it is an ethylene-propylene-diene-monomer terpolymer which, in turn, is usually referred to as an "EPDM". Such cables are disclosed, for example, in U.S. Pat. No. 4,088,830, and typical insulation formulations compounded from such EPDM polymers together with additional constituents, such as suitable processing aids, curing agents and the like, have been shown, for example, in U.S. Pat. No. 3,926,900.
In formulating compounds for such ethylene-propylene rubber compositions, it has been necessary to strike a balance between the desire for compounds having better physical characteristics, i.e. modulus, hardness etc., and the counterbalancing highly undesirable changes in other physical properties, such as, for example, an increase in the coefficient of thermal expansion. Thus, it was possible in the past, by employing an EP rubber with a very high ethylene to propylene ratio, to obtain some improvement in modulus and hardness. Unfortunately, such EP rubbers also exhibited a substantially higher coefficient of thermal expansion. When used in cables, the high coefficient of thermal expansion resulted in very high stress levels being placed on the outer lining or jacket of lead, with the result that splits and cracks in the lead could result, allowing well fluids and gases into underlying insulation.
Nevertheless, it has long been desired to formulate an EP rubber based compound which would have higher modulus and hardness so that the resulting insulation would be more resistant to mechanical deformation at room temperature. Such insulation deformation frequently occurs during the cable armoring process, as well as when the cable is handled during service.
Suitable EP rubbers and EPDM polymers are available from a variety of sources including Uniroyal under the trademark ROYALENE and E. I. Du Pont under the trademark NORDEL. The final formulation or "compound", however, typically will also include curing agents, processing aids, fillers, pigments and/or cross-linking agents. In general, the selection of a particular curing system, or other additive, is usually a matter of choice for the individual formulator subject to certain well-known characteristics attributable to specific additives.
Again, as noted above, these characteristics often involve something of a tradeoff. For example, it is generally accepted in EP rubber compounding that higher modulus compounds can be obtained by using carbon black or precipitated silica fillers. However, these fillers are unsuitable for cable applications because they seriously degrade the electrical properties. In other words, even though it is known that compounds having the desired higher modulus could be obtained using carbon black or precipitated silica fillers, in cable applications, it has been traditional to use surface treated clay as the filler because the electrical properties are the principal properties which must be maintained.
It has long been desired to produce a cable with a multi-layer covering over the conductor wherein the thermosetting rubber insulation has not only excellent electrical insulation properties, but also improved physical properties, particularly higher modulus and hardness, so as to provide greater protection against deformation of the insulation during armoring of the cable within a metal shield and/or during handling of the cable in service.
Accordingly, it is a principal object of the present invention to provide an electrical cable of the type used in oil and gas wells which has excellent physical and chemical properties.
In addition, other objects of the invention will become apparent to those skilled in the art from a reading of the following specification and claims.